This invention relates to a fuel cell system and more particularly to a system having a plurality of cells which consume an H2-rich gas to produce power.
Fuel cells have been used as a power source in many applications. For example, fuel cells have been proposed for use in electrical vehicular power plants to replace internal combustion engines. In proton exchange membrane (PEM) type fuel cells, hydrogen is supplied to the anode of the fuel cell and oxygen is supplied as the oxidant to the cathode. PEM fuel cells include a membrane electrode assembly (MEA) comprising a thin, proton transmissive, non-electrically conductive solid is polymer electrolyte membrane having the anode catalyst on one of its faces and the cathode catalyst on the opposite face. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain appropriate channels and/or openings therein for distributing the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode catalysts. The term fuel cell is typically used to refer to either a single cell or a plurality of cells (stack) depending on the context. A plurality of individual cells are commonly bundled together to form a fuel cell stack and are commonly arranged in series. Each cell within the stack comprises the membrane electrode assembly (MEA) described earlier, and each such MEA provides its increment of voltage. A group of adjacent cells within the stack is referred to as a cluster. Typical arrangements of multiple cells in a stack are described in U.S. Pat. No. 5,763,113, assigned to General Motors Corporation.
In PEM fuel cells, hydrogen (2) is the anode reactant (i.e., fuel) and oxygen is the cathode reactant (i.e., oxidant). The oxygen can be either a pure form (O2), or air (a mixture of O2 and N2), or O2 in combination with other gases. The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprises finely divided catalytic particles, which are often supported on carbon particles, and mixed with a proton conductive resin. The catalytic particles are typically costly precious metal particles. These membrane electrode assemblies are relatively expensive to manufacture and require certain conditions, including proper water management and humidification, and control of catalyst fouling constituents such as carbon monoxide (CO), for effective operation.
For vehicular applications, it is desirable to use a liquid fuel such as an alcohol (e.g., methanol or ethanol), or hydrocarbons (e.g., gasoline) as the source of hydrogen for the fuel cell. Such liquid fuels for the vehicle are easy to store onboard and there is a nationwide infrastructure for supplying liquid fuels. However, such fuels must be dissociated to release the hydrogen content thereof for fueling the fuel cell. The dissociation reaction is accomplished within a chemical fuel processor or reformer. The fuel processor contains one or more reactors wherein the fuel reacts with steam and sometimes air, to yield a reformate gas comprising primarily hydrogen and carbon dioxide. For example, in the steam methanol reformation process, methanol and water (as steam) are ideally reacted to generate hydrogen and carbon dioxide. In reality, carbon monoxide and water are also produced. In a gasoline reformation process, steam, air and gasoline are reacted in a fuel processor which contains two sections. One is primarily a partial oxidation reactor (POX) and the other is primarily a steam reformer (SR). The fuel processor produces hydrogen, carbon dioxide, carbon monoxide and water. Downstream reactors may include a water/gas shift (WGS) and preferential oxidizer (PROX) reactors. In the PROX, carbon dioxide (CO2) is produced from carbon monoxide (CO) using oxygen from air as an oxidant. Here, control of air feed is important to selectively oxidize CO to CO2.
Fuel cell systems which process a hydrocarbon fuel to produce a hydrogen-rich reformate for consumption by PEM fuel cells are known and are described in co-pending U.S. Pat application Ser. Nos. 08/975,422 and 08/980,087, filed in November, 1997, and U.S. Ser. No. 09/187,125, filed in November, 1998, and each assigned to General Motors Corporation, assignee of the present invention; and in International Application Publication Number WO 98/08771, published Mar. 5, 1998. A typical PEM fuel cell and its membrane electrode assembly (MEA) are described in U.S. Pat. Nos. 5,272,017 and 5,316,871, issued respectively Dec. 21, 1993 and May 31, 1994, and assigned to General Motors Corporation.
Efficient operation of a fuel cell system depends on the ability to provide effective water management in the system and specifically to control the recovery and recycling of water in the system.
A fuel cell system produces water as a product of the electrochemical reaction that occurs in a fuel cell stack. The physical state of the product water depends on the temperature and pressure at which the electrochemical reaction occurs. It can be generally stated that the product water will be vapor at higher temperatures and lower pressures, and liquid at lower temperatures and higher pressures. Therefore, it is possible that the product water exist as liquid when the fuel cell stack is cool, and gradually transition to water vapor when the stack reaches full operating temperature.
It is necessary to continually recover the product water so that it may be used for other purposes within the fuel cell systems such, for example, as to provide water to the fuel processor as a reactant. The water is recovered differently depending on the physical state. When in liquid form, the product water is typically recovered by a mechanical water separator, and when the product water is vapor it is typically recovered by a condenser. This invention relates to the mechanical water separator to recover the liquid water when present.
Design of a mechanical liquid water separator presents a tradeoff between separating efficiency, gas flow pressure drop, and physical volume. The objective is to maximize separating efficiency, minimize gas flow pressure drop, and minimize physical volume of the component. Maximum separating efficiency is desired so that sufficient product water is recovered for other uses in the system. Minimal pressure drop is desired to minimize the power requirements in the system, thus increasing overall system efficiency. Minimum physical volume is desired so that the component may be easily packaged in an automotive fuel cell application.
Currently available industrial water separators are designed for specific, dedicated gas flow rates and water loading. Because gas flow rates and liquid water loading in a fuel cell system are not constant, current separator designs must be sized for the highest gas flow rates and worst water loading possible. The consequence of this is an over-designed component that experiences the maximum gas flow and/or water loading only for a small percentage of time (i.e. start-up conditions). The over-designed component provides the fuel cell system with high pressure drop (inefficiency), or large physical volume, which makes it non-useful in an automotive fuel cell application.
The present invention overcomes the problem described. It provides the ability to separate liquid water when it exists and it allows gas (including water vapor) to freely flow through when no liquid water exists. The advantage is that the pressure drop in the device is only recognized by the system when the component is actively functioning. When there is no need for the component, due to the lack of liquid water, the system does not recognize its presence thus increasing overall system efficiency. Also, there is an opportunity for the device to be a smaller volume because it is functional for only a small fraction of the time. That is, the smaller volume will generate higher pressure drop but the impact to system efficiency will be minimized because the devices functional for a small percent of the time. Furthermore, when liquid water does exist it is possible to control the separation efficiency according to water recovery requirements.
According to the invention, the separator comprises a housing defining an inlet, an outlet, and a longitudinal flow through passage extending between the inlet and the outlet; a plurality of baffles arranged in longitudinally spaced serial relation in and along the passage and each blocking a portion of the cross section of the passage; and means mounting the baffles for relative movement to selectively vary the composite cross-sectional blockage defined by the baffles. With this arrangement, the individual baffles may be selectively adjusted to selectively vary the composite cross-sectional blockage defined by the baffles and thereby customize the separator to accommodate the specific gas flow rates and water loading being instantaneously encountered in the system.
According to a further feature of the invention, each baffle is mounted for rotation about a central axis extending through the passage. This specific arrangement facilitates the relative adjustment of the baffles to accommodate a particular water loading scenario.
According to a further feature of the invention, each baffle includes a central hub positioned on the axis and a plurality of circumferentially spaced vanes extending radially outwardly from the hub. This specific arrangement further facilitates the ability to readily adjust the separator to accommodate a specific water loading scenario.
According to a further feature of the invention, the vanes of each baffle block a given percentage of the cross-sectional area of the passage so that first and second successive baffles in the passage may be relatively rotated to define a composite cross-sectional blockage varying from the given percentage of the cross-sectional passage area to a larger percentage of the cross-sectional passage area. For example, the vanes of each baffle may block approximately one half of the cross-sectional area of the passage so that the first and second successive baffles in the passage may be relatively rotated to define a composite cross-sectional blocking varying from approximately one half of the cross-sectional passage area to essentially all of the cross-sectional passage area.
According to a further feature of the invention, the separator further includes means operative to relatively move the baffles in response to sensed system parameters.
According to a further feature of the invention, the moving means comprises an actuator drivingly associated with the baffles and means operative to generate a control signal for delivery to the actuator reflecting sensed parameters of the fuel cell. This arrangement allows the separator to adjust automatically to accommodate the instantaneous water loading situation being encountered in the fuel cell system.
According to a further feature of the invention, the sensed parameters of the fuel cell include one or more of the humidity of a compressed gas stream delivered to the fuel cell, the mass flow rate of the gas stream, the temperature and pressure within the fuel cell, and the electrical current produced by the fuel. These parameters in various combinations provide a measure of the instantaneous water loading status of the fuel cell and thereby provide a means of generating a signal that allows the separator to adjust to reflect the instantaneous water loading status of the system.
According to a further feature of the invention, the separator includes a central shaft positioned on the axis, and the central hubs are annular and are fitted over the shaft in serial stacked relation. This arrangement allows the baffles to be provided in modular form so to as to be readily added to or subtracted from the system to adjust the system to varying water loading scenarios.
According to a further feature of the invention, the housing is sectional and includes modular components which may be interchanged to vary the size of the housing in accordance with the number of baffle modules employed. This arrangement allows the overall separator assembly to be readily customized to the specific water loading conditions being encountered.
According to a further feature of the invention, the actuator rotatably drives the shaft and selected hubs of selected modular baffles are keyed to the shaft so that rotation of the shaft by the actuator relatively rotates the baffles. This arrangement provides a ready and efficient means of relatively moving the baffles to vary the flow characteristic of the separator in accordance with the signal being received by the actuator.
According to a further feature of the invention, the separator is arranged to receive the cathode effluent from the fuel cell and functions to separate the water from the cathode effluent for delivery as a water reactant to the fuel processor.
According to a further feature of the invention, a separator as described is also provided to receive the anode effluent from the fuel cell and/or to receive the reformate upstream of the fuel cell. These further separators provide a further means of effectively removing water from the system for delivery to the fuel processor as a water reactant.
In an alternate embodiment of the invention, the means for relatively moving the baffles comprise bimetal means drivingly associated with selected baffles and operative to relatively move the baffles in response to environmental temperature variations. This arrangement provides a passive system for controlling the relative positions of the baffles as opposed to the active system provided by the actuator driving a shaft mounting the baffles.